The Collections Class

The Java Collections Framework does not only provide data structures (interfaces and implementation classes). It also provides a highly powerful utility class named java.util.Collections.

[!WARNING] Do not confuse the Collection interface with the Collections utility class!

• Collection: The root interface of the framework.
• Collections: A utility class containing static methods to operate on collections.

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Important Static Methods

The Collections class consists exclusively of static methods that operate on or return collections. It contains polymorphic algorithms that operate on collections, "wrappers" (which return a new collection backed by a specified collection), and a few other odds and ends.

1. Collections.sort()

Used to sort the elements present in the specified list of collections in ascending order.

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class LabCollections1 {

  public static void main(String args[]) {
    List<String> list = new ArrayList<>();

    list.add("Zebra");
    list.add("Apple");
    list.add("Mango");

    System.out.println("Before sorting: " + list);

    // Using the Collections utility class to sort the list
    Collections.sort(list);

    System.out.println("After sorting: " + list);
  }
}

Output:

Before sorting: [Zebra, Apple, Mango]
After sorting: [Apple, Mango, Zebra]

2. Collections.reverse()

Reverses the order of the elements in the specified list.

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class LabCollections2 {

  public static void main(String args[]) {
    List<Integer> list = new ArrayList<>();
    list.add(1);
    list.add(2);
    list.add(3);

    // Reverses the current order
    Collections.reverse(list);

    System.out.println("Reversed List: " + list); // [3, 2, 1]
  }
}

3. Collections.binarySearch()

Searches the specified list for the specified object using the binary search algorithm.

[!IMPORTANT] The list must be sorted in ascending order (using Collections.sort()) prior to making this call. If it is not sorted, the results are undefined.

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class LabCollections3 {

  public static void main(String args[]) {
    List<Integer> list = new ArrayList<>();
    list.add(50);
    list.add(10);
    list.add(30);

    // 1. Sort the list first
    Collections.sort(list); // List is now [10, 30, 50]

    // 2. Perform Binary Search
    int index = Collections.binarySearch(list, 30);

    System.out.println("Element 30 found at index: " + index); // 1
  }
}

4. Collections.max() and Collections.min()

Returns the maximum or minimum element of the given collection, according to the natural ordering of its elements.

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class LabCollections4 {

  public static void main(String args[]) {
    List<Integer> list = new ArrayList<>();
    list.add(100);
    list.add(5);
    list.add(2000);

    System.out.println("Maximum: " + Collections.max(list)); // 2000
    System.out.println("Minimum: " + Collections.min(list)); // 5
  }
}